Transcript Basic Concepts

Programming Language
Concepts
Formal Syntax
Paradigms
Data Types
Polymorphism
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General Issues
Representation (of data, of computation)
Form (Syntax)
Meaning (Semantics)
Paradigm (General way of thinking)
Naming (names, name spaces, bindings, locality)
Functionality (numeric, data manip, I/O,
communication, synchronization, security)
Correctness (types, exception handling, error
checking, bug avoidance)
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Syntax
Syntax: The grammatical form of programs.
vs Semantics: The meaning of the program
Syntax (of textual languages) is typically specified
by production rules for a context-free grammar
using Backus-Naur Form (BNF) or Extended BNF
(EBNF)
In visual languages, syntax is described by a set
of restrictions on how diagrams may be
constructed. (e.g., connection constraints)
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Syntactic Components
Identifiers and reserved words
Numeric constants
Parentheses, braces and brackets
Expressions
Statements
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BNF (Backus-Naur Form)
(2.0 * PI) / n
<expression>
<term>
<factor>
::=
<expression> + <term>
|
<expression> - <term>
|
<term>
::=
<term> * <factor>
|
<term> / <factor>
|
<factor>
::=
number
|
name
|
( <expression> )
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Extended BNF
Optional constructs written as [ x ]
Zero or more of x written as { x }
Choice (“or”) written using |
Grouping with parentheses ( x | y ) as in { (x | y ) z }
<expression> ::= <term> { (+ | -) <term> }
<term>
::= <factor> { (* | /) <factor> }
<factor>
::= ’(’ <expression> ’)’ | name | number
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Derivation
E
::=
E + T
|
E - T
T
::=
T * F
|
T / F
F
::=
number
|
name
|
T
|
|
F
( E )
E
F / F + F
E + T
25 / F + F
T + T
25 / 100 + F
T / F + T
25 / 100 + total
F / F + T
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Representation of Data
Constants, Variables
Types, classes
Compounds: arrays, structures.
Non-numeric objects: strings, images, audio.
Values vs references
Machine dependencies: word size, addressing
resolution.
In C, characters and booleans are actually integers.
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Representation of Process
Arithmetic and logical expressions
Conditional expressions
Loops
Recursive and nonrecursive functions
Multiple threads of control, forking, joining,
synchronizing
Single-threaded Parallel processing (in Singleinstruction stream/multiple data stream processors)
Throwing and catching of exceptions
Declaration of constraints and rules
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Paradigm
General style of thinking that underlies a
programming language
Webster’s New World Dictionary: “a pattern,
example, or model”.
Imperative
Rule-based
Functional
Logic
Object-oriented
Visual data-flow
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The Imperative Paradigm
An imperative program is a sequence of commands
Read a value from a file.
Evaluate an arithmetic expression.
Assign a value to a variable.
Test a condition and branch if it is true.
Iterate a loop body until a condition is false.
Print a value onto the screen.
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The Functional Paradigm
An functional program is a collection of function definitions
and function applications.
Define SQR(x): {
Apply the * function to x and x}
Apply SQR to 7;
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The Object-Oriented Paradigm
An object-oriented program is a collection of object class
definitions, in which both data members and methods are
specified.
Class Student extends Person {
int student_number;
int get_student_number() { return student_number; }
int set_student_number (int num) {
student_number = num;
}
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The Rule-Based Paradigm
A rule-based program is a collection of if-then rules.
if name = "" then input name;
if name starts with "A" then print "Early in the
alphabet";
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The Logic-Programming
Paradigm
A logic program is a collection of logical propositions and
questions.
If x is a bird or an airplane, then x has wings.
Tweety is a bird.
Does Tweety have wings?
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The Visual Data-Flow Paradigm
A visual data-flow program is a diagram in which boxes
represent operations and arrows indicate the flow of data
from outputs of operations to inputs of other operations.
3x2 + 5x + 8
3
*
*
input
x
+
*
+
5
8
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Types
Category or class for a value (or object) that
permits its bits to be interpreted.
Central Processing Unit instruction sets
recognize certain types, such as integers and
floats of different sizes.
A programming language is not limited to the
types that are directly supported by the CPU.
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Strong vs Weak Typing
Strong typing:
Every variable must be declared with a type.
A variable may receive only values of its type.
Type-related bugs can be reduced.
Type identification tags are not needed a run time.
Weak typing:
Variables need not be declared with particular types.
The type of value held by a variable can change dynamically.
Values must be tagged during execution with their types.
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Coercion and Contagion
Coercion: Any automatic type conversion.
A value of type A is being assigned to a variable of type B.
The value is coerced into one of type B.
double x = 3 * 5;
Contagion:
Values of lower precision “catch” the precision of their higher
precision co-arguments.
int y = 10 * (3.1415 / 10);
/* result is 3, not 0. */
In Common Lisp, when a rational meets a float, the rule of floatingpoint contagion rules.
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Type Inference
With weak typing, type inference is needed.
The resulting type can be determined from the types of the
arguments, and the nature of the operations.
Contagion provides one method for type inference:
With strong typing, means for determining type
equivalence are needed.
int x[10];
int y[10];
Here x and y have structurally equivalent types.
C and C++ recognize structurally equivalent types as equivalent.
In Modula, most structurally equivalent types are not automatically
considered equivalent, but they can be declared equivalent.
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Polymorphism
The ability for a function or a variable to receive values
of different types.
(setq x 5)
(setq x ‘(a b c))
In Perl, is this polymorphism?
$x = 5;
$x = "This is a string. ";
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Polymorphism in Java
Vector v = new Vector( );
v.addElement(5);
v.addElement("A string");
Generality of type can be achieved by declaring formal
parameters to be of class Object.
Vector returnTwo(Object obj) {
Vector pair = new Vector( );
pair.addElement(obj);
pair.addElement(obj.clone());
return pair;
}
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Benefits of Polymorphism
One polymorphic function does the work of several
more specific functions.
(push element)
(push-integer 5)
(push-string
"string")
A polymorphic variable can hold a piece of data in
different forms without requiring additional variables.
(setq president ’(abe lincoln))
(setq president (format nil "~A" president ))
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